Homeothermic animals, such as humans, strive to maintain an internal or core body temperature that is within a comparatively narrow range regardless of the effects of external conditions or internal stresses. In humans, the thermal core generally includes the vital organs of the body, such as the brain and the several organs maintained within the abdomen and chest. Peripheral tissues, such as the skin, fat, and muscles, act as a buffer between the thermal core and the external environment of the animal by maintaining a temperature gradient that ranges from near-core temperature within internal tissues to near-ambient temperature at the surface of the animal.
Generally, the body compensates for minor fluctuations in body temperature through an autonomic thermoregulatory control mechanism that is triggered by temperature sensors located both in the body core as well as on the surface of the skin. For example, when the body experiences an upward shift in temperature, an involuntary response referred to as perspiration attempts to dissipate the excess heat by transferring moisture from body tissues to the body surface. Once at the surface, the moisture evaporates and carries with it some quantity of stored heat. Conversely, when experiencing a downward shift in temperature, the body attempts to generate heat by shivering, an involuntary contraction and expansion of muscle tissue throughout the body which creates heat through friction.
Hypothermia is a condition experienced by a homeothermic animal when the body is unable to generate sufficient heat to overcome external temperature conditions and the animal's core temperature falls below the normal range for the species. In extreme cases, a lowered core temperature renders it impossible for the body to maintain normal bodily functions. In humans, hypothermia may be defined as a core temperature of less than about 35° Celsius. Hypothermia may be caused when the body is overwhelmed by low environmental temperatures or is otherwise compromised, such as by being placed under general anesthesia, which induces a loss of the ability to conserve bodily heat. Hypothermia generally results in several physiological responses which attempt to prevent further heat loss. The most important of these responses is vasoconstriction, a constriction of the peripheral blood vessels which limits blood flow to the extremities and therefore reduces heat transfer away from the thermal core of the body. While vasoconstriction appropriately limits heat loss from the thermal core, it also makes it much more difficult to reverse a hypothermic state by applying heat to the surface of the body. That is, in restricting heat transfer from the core to the periphery of the body, vasoconstriction also impedes the transfer of heat from the body surface to the thermal core. This physiological impediment to heat transfer is referred to as a vasoconstrictive blockade to heat exchange.
U.S. Pat. No. 5,683,438, issued to Grahn and assigned to Stanford University, discloses an apparatus and method for overcoming the vasoconstrictive blockade to heat exchange by mechanically distending blood vessels in a body portion and providing for the transfer of heat to the body core of a hypothermic mammal. The disclosed device comprises a fluid-filled heating blanket that is lodged within a tubular, elongated sleeve which is placed over the body portion. The sleeve is maintained around the body portion of the patient by means of a flexible, tubular flange which seals around the body portion when sub-atmospheric pressure is applied and maintained within the sleeve. This system may be improved in several regards. First, more adequate means for securely fastening the device around body portions of varying diameter may be provided. Second, means in addition to the application of a vacuum within the sleeve may be provided to ensure optimal contact between the heating element and the body portion during treatment. Further, means for storing thermal energy within the sleeve may be provided so that the unit may be disconnected from the heat source without significant heat loss during patient transport.
In view of the foregoing, a need exists for an apparatus and method for manipulating the core temperature of a mammal which overcomes the shortcomings of the prior art. Accordingly, there is a need for an apparatus which adapts to the variability in patient sizes. There is also a need for a device which provides a means for securing the device around a body portion of a patient such that the body portion is inhibited from moving relative to the device during treatment. There is a further need for a device which optimizes contact between the heating element and the body of the patient, thereby optimizing heat transfer. Additionally, there is a need for a device which is capable of storing thermal energy such that the unit may be disconnected from the heat source without significant heat loss during patient transport.